Rats which have been exposed to carbon monoxide to the point of respiratory arrest at 5 days of age, postnatally, develop locomotor hyperactivity which persists until they are about 3 months old. Visualization of the neurons of the caudate nucleus using the rapid Golgi method shows changes in the morphology of the neurons during this period. As the maturing rats become less hyperactive, the number of dendritic spines on the caudate neurons increases. The effects of neonatal carbon monoxide on activity are being compared with the effects of electrolytic lesions of the globus pallidus. Additional experiments will evaluate the portion of the 24-hour diurnal cycle in which hyperactivity is most pronounced by having the lesioned rats live in a residential maze equipped with photocells for recording activity throughout the day. Amphetamine will be injected during the 12-hour diurnal and 12-hour nocturnal portions of the day to examine the effect of the stimulant on lesion-induced hyperactivity at different portions of the diurnal cycle. BIBLIOGRAPHIC REFERENCES: Culver, B., and Norton, S. Juvenile hyperactivity in rats after acute exposure to carbon monoxide. Exp. Neurol. 50: 80-98, 1976. Norton, S. Hyperactive behavior of rats after lesions of the globus pallidus. Brain Res. Bull. 1: 193-202, 1976.